The innate immune system provides the body with a first line defense against invading pathogens. In an innate immune response, an invading pathogen is recognized by a germline-encoded receptor, the activation of which initiates a signaling cascade that leads to the induction of cytokine expression. Innate immune system receptors have broad specificity, recognizing molecular structures that are highly conserved among different pathogens. One family of these receptors is known as Toll-like receptors (TLRs), due to their homology with receptors that were first identified and named in Drosophila, and are present in cells such as macrophages, dendritic cells, and epithelial cells.
There are at least ten different TLRs in mammals. Ligands and corresponding signaling cascades have been identified for some of these receptors. For example, TLR-2 is activated by the lipoprotein of bacteria (e.g., E. coli.), TLR-3 is activated by double-stranded RNA, TLR-4 is activated by lipopolysaccharide (i.e., LPS or endotoxin) of Gram-negative bacteria (e.g., Salmonella and E. coli O157:H7), TLR-5 is activated by flagellin of motile bacteria (e.g., Listeria), TLR-7 recognizes and responds to imiquimod (and ssRNA) and TLR-9 is activated by unmethylated CpG sequences of pathogen DNA. The stimulation of each of these receptors leads to activation of the transcription factor NF-κB, and other signaling molecules that are involved in regulating the expression of cytokine genes, including those encoding tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and certain chemokines. Agonists of TLR-7 are immunostimulants and induce the production of endogenous interferon-α in vivo.
There are a number of diseases, disorders, and conditions linked to TLRs such that therapies using a TLR agonist are believed promising, including but not limited to melanoma, non-small cell lung carcinoma, hepatocellular carcinoma, basal cell carcinoma, renal cell carcinoma, myeloma, allergic rhinitis, asthma, COPD, ulcerative colitis, hepatic fibrosis, and viral infections such as HBV, HCV, HPV, RSV, SARS, HIV, or influenza.